The present invention relates to a method for transmitting telegrams in an automation system. The present invention also relates to an automation system.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Real-time systems are preferably used as the communications link in automation systems since these guarantee excellent performance in relation to communication and high availability. In some cases the data is relevant not only to the individual network components of the automation system, but to a range of applications. Communication between the individual network components of an automation system which brings the required data to its destination with the least possible delay would here be ideal. A natural limit is given here by the data rate of the communications link. The delay in data transmission should not be critically limited by additional influencing variables, however. This should also apply if individual network components, which are located on the path between source and destination, fail.
In automation systems data are transmitted between the individual network components in the form of telegrams by way of example. Field buses, which are defined in the standard IEC 61158, are conventionally used as data lines. Ethernet connections by way of example are used for this purpose and these comprise appropriate switches, in which the data are channeled directly from the source to the destination with the aid of the address formations at the start of the telegram. A minimum delay of the switches must here be included. To be able to guarantee corresponding decoupling in the event of a failure what is known as the “store-and-forward principle” is usually used which buffers the individual telegrams in each network component. This results in a total transmission time which is conventionally in the range of a few milliseconds. An Ethernet connection is basically an active network in which the required availability in the event of a failure of one of the network components or a network segment is achieved by a redundant path selection. This is structurally achieved by way of example in that a linear topology is combined to form a ring.
In a series of applications one data source responds to a plurality of data sinks (Multicast). This is the case for example if the measured value for a current is required for various protective functions. Such a multicast communications link assumes that for reasons of efficiency the data is only transmitted once. A further important aspect is the synchronization of the data. If the data are temporally distorted then the result of a link is doubtful. It is likewise necessary that non-time-critical data can also be transmitted on the same communications channel.
Other real-time systems such as EtherCAT use a packetized transmission. However, a multicast connection may only be achieved starting from a master. The sequence plays here also a decisive role due to the direction dependency.
Many real-time systems such as Profinet reserve a certain data rate for cyclical services. The remaining data can then be exchanged via a standard Ethernet connection. The efficiency of the data transmission does not change in principle therefore, but the minimum cycle times are limited in exchange. A further problematical aspect is what is known as the Frame Overhead.
In a client-server communication relationship this is solved by a dynamic packetizing behavior in that a selected component is either the source or the sink of the data. With multicast connections an individual transmission can also conventionally be used, with both the data transmission rate and the system load being hereby limited.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved automation system with a plurality of network components, which allows a more effective data transmission between the individual network components.